1. Field of Invention
This invention relates to heat exchangers, and more particularly to heat exchangers in which heat is indirectly transferred between chemically reactive or otherwise incompatible substances.
2. Description of the Prior Art
Shell and tube type heat exchangers are often used to transfer heat between fluids which are known to be chemically or physically reactive with each other. Direct contact between these reactive fluids due to leakage within the heat exchanger causes highly undesirable effects including the formation of corrosive products which may destroy the materials of the heat exchanger, exothermic chemical reactions resulting in locally elevated temperatures at the location of the leak which may reduce the material strength at this location leading to progressive enlargement of the leak, and the production of gaseous and/or corrosive reaction products which cause undesirable pressure fluctuations or explosions or corrosion of nearby surfaces substantially damaging or destroying the heat exchange apparatus.
It is known that the aforementioned effects may be minimized by utilizing a coaxial tube arrangement, in which the intermediate volume formed between the coaxial tubes provides an annular space for a heat conductive substance not reactive with either of the reactive fluids. The intermediate substance may serve as either a diluent for fluids which leak into the intermediate volume or as an indicator of leakage. Depending upon the fluids employed, the presence of leakage may be indicated by various physical or chemical manifestations, for example, changes in pressure, temperature, viscosity, color, odor or electrical conductivity.
Illustratively, in some nuclear power plant designs, a liquid metal such as liquid sodium is used to cool the reactor and transfer heat to water for the generation of steam or to superheat steam. Liquid sodium, which reacts violently when it comes into contact with water or steam producing highly metal-corrosive substances and hydrogen gas, is separated from the water or steam in the steam generating or superheating heat exchangers by a tube bundle of radially spaced coaxial tubes. Typically, in the prior art, liquid sodium flows through the inner tubes and a flow of water or steam is maintained over the outside surface of the tube bundle. The annular space between the inner tubes and the coaxial outer tubes is filled with another intermediate heat conductive liquid which does not react with either sodium or water in an undesirable fashion, e.g., mercury. The intermediate fluid is provided by a system which includes means of fluid communication with the annular spaces and suitable sight glasses, level indicating devices or pressure indicators. The pressure of the liquid sodium may be greater or less than that of the intermediate fluid. Any leak in an inner or outer tube results in a change in the level or pressure of the intermediate fluid, thereby giving an indication of such leakage. Upon such an indication, the heat exchange apparatus may be shut down, drained and repaired without having experienced the undesirable consequences of contact between the reactive substances. It is apparent to those skilled in the art that this configuration, while effective, is difficult to analyze for stresses, expensive to construct, and often less efficient in heat transfer due to the presence of an intermediate heat transfer substance which may have lower thermal conductivity than the substance in which it is contained and through which the heat must pass.